Battery Calculator 2025: How to Size Your Home System and Save
Thinking About a Home Battery in 2025? Here’s How to Calculate the Right Size and ROI
With battery rebates rolling out across Australia in 2025, homeowners everywhere are asking the same questions:
- How big should my battery be?
- Will it power my home through a blackout?
- Will it pay for itself?
In this guide, we walk through simple and advanced battery calculators to help you understand the ideal size for your home battery, how to calculate your return on investment, and what you can expect from modern solar and battery proposals.
Why Battery Size Matters
You don’t want a battery that’s too big—it may never fully charge from your solar system. But you also don’t want one too small that runs out of power before dinner. The key is to match your battery size to your solar output and your energy use across the day and night.
Quick Battery Sizing Rule
Most Aussie homes consume around 20kWh of electricity daily. That typically breaks down to:
- 5kWh during the day
- 15kWh at night
A simple rule: divide your daily consumption (20kWh) by 4. This gives you a recommended 5kW solar system. It will generate about 20kWh/day (5kW x 4 sun hours):
- 5kWh goes to your daytime usage
- 15kWh charges the battery for use at night
Using Advanced Battery Calculators
For larger homes or higher usage, you might need to go deeper. Estimate your average daily consumption by checking your electricity bill and dividing total kWh by the number of days in the billing cycle. Then break that down into daytime and nighttime percentages (e.g. 25% day, 75% night).
If your average daily use is 40kWh, and 30kWh is at night, you’ll want:
- 10kWh for daytime offset
- 30kWh battery for night
- At least a 10kW solar system (ideally more to cover seasonal variations)
Planning for Backup Power
Batteries are also valued for backup in blackouts. To calculate how much storage you need:
- Light (100W x 5h) = 0.5kWh
- Fridge (2kW x 5h) = 10kWh
Add up all devices you want on backup. If your total comes to 10.5kWh, you’ll want a battery that’s at least 12kWh so you’re not fully draining it every time.
Do You Want to Trade on a VPP?
Virtual Power Plants (VPPs) let you sell excess stored energy during peak times. If you allocate 20% of your 10kWh battery (2kWh) to the VPP, you’ll have 8kWh for personal use. Retailers control this based on forecasts and may offer revenue in return.
Reading a Solar + Battery Proposal 10kW solar + 10kWh battery
- Estimated generation: ~56kWh/day in summer, ~26kWh/day in winter
- This averages ~40kWh/day annually.
Expected Annual Savings
Savings come from:
- Solar self-consumption during the day
- Battery offset during peak evening rates
- VPP or grid services like FCAS
Example:
- Day usage offset: 15kWh x $0.50 = $7.50
- Battery offset: 10kWh x $0.50 = $5.00
- Grid feed-in: 15kWh x $0.05 = $0.75
- Total: $13.25/day x 365 = ~$4,836/year
Adjusted for real-world pricing (~$0.35/kWh), total savings are closer to $3,200/year.
What About ROI?
Let’s say your system costs $24,000 upfront. With rebates ($3,300 solar + $3,800 battery), you’ll pay ~$17,000 out of pocket.
- Savings: $3,200/year
- Payback: ~5.3 years
- IRR: 11%
Financing at $0 upfront? Monthly repayments can match your current energy bill, meaning no added expense.
Bonus: At the end of the term, the system adds resale value to your home.
Ready to Size Your System?
Use our free battery calculator to:
- Find your ideal solar + battery size
- Estimate savings
Model backup and VPP capacity
And get 3 no-obligation quotes from trusted local installers now here
Investing in solar and batteries is no longer just about the environment—it’s a smart financial move.
Cost of Solar Battery Systems in 2025 (Battery Calculator Guide)
Components of a solar battery system cost
A home battery system involves several cost components:
- Battery modules: The core storage unit. Battery costs range from $7 000–$12 000 for a 10 kWh system before rebates. Larger systems lower the cost per kWh because fixed costs are spread over more capacity.
- Hybrid or AC inverter: If your existing solar system is not “battery‑ready,” you’ll need an inverter to manage battery charging and discharging. Hybrid inverters typically cost $2 000–$3 000.
- Installation and upgrades: Labour, wiring, switchboard upgrades and meter changes add $1 500–$3 500. Difficult installations (tight spaces, longer cabling) can increase costs.
- Monitoring, permits and compliance: Some states require inspection fees or additional safety devices. Monitoring software is often included, but premium packages may cost extra.
Average installed prices by capacity
Solar Choice’s Battery Price Index (August 2025) provides indicative installed prices after the federal rebate for systems between 5 kWh and 20 kWh. The table below summarises typical battery‑only costs and battery‑plus‑inverter costs. (All figures include GST and federal rebates.)
| Usable capacity | Battery‑only price | Battery‑plus‑inverter price | Approx. cost per kWh* |
| 5 kWh | ≈$5 180 | ≈$6 580 | $1 036–$1 316 |
| 10 kWh | ≈$8 260 | ≈$9 860 | $826–$986 |
| 15 kWh | ≈$11 190 | ≈$13 140 | $746–$876 |
| 20 kWh | ≈$14 120 | ≈$16 920 | $706–$846 |
*Cost per kWh decreases with larger batteries because many installation costs are fixed.
Brand‑specific price examples
Penrith Solar Centre (PSC Energy) lists installed prices after rebates for popular batteries in 2025:
- Enphase IQ Battery 5P (two units for 10 kWh): ≈$11 990. Enphase batteries use micro‑inverters built into each module for added redundancy, which increases cost but improves reliability.
- Tesla Powerwall 3 (13.5 kWh): ≈$12 500. The Powerwall includes a hybrid inverter; when installed with a new solar system, its two‑in‑one design can reduce hardware and labour costs.
- Sigenergy SigenStor (8 kWh): starting at $10 500 for battery and inverter blocks, with extra 8 kWh modules around $4 990 each. SigenStor’s modular design reduces installation labour and scales well.
- DC‑coupled system costs: When paired with a new PV array, Sigenergy’s system starts around $16 000 and a Powerwall 3 system around $18 500. DC coupling improves efficiency but requires installing solar and battery together.
Factors influencing cost
- System size: A larger battery yields lower per‑kWh cost but increases the total outlay. Over‑sizing can mean wasted capacity; under‑sizing may require grid power at night.
- Retrofit vs. new installation: Adding a battery to an existing solar system often requires an AC‑coupled setup, adding cost and complexity. Installing solar and battery together allows DC coupling and can be cheaper overall.
- Brand and technology: Premium systems (Tesla, Enphase) cost more but offer features like integrated inverters, advanced apps and longer warranties. Budget or modular systems (Alpha‑ESS, BYD, SigenStor) provide flexibility at lower price points.
- Location and labour: Prices vary by state and installer. Rural installations or complex wiring can increase labour costs.
- Rebates and incentives:
- Federal rebate: About $345 per usable kWh (≈30 % off) for batteries installed from 1 July 2025. Only batteries between 5 kWh and 100 kWh installed by CEC‑accredited installers qualify.
- NSW VPP incentive: Roughly $550 for 10 kWh and $1 500 for 27 kWh batteries. Must join an approved VPP.
- WA Residential Battery Scheme: Rebates up to $3 800 and interest‑free loans up to $10 000 for 5 kWh+ systems.
- Note: Victoria’s battery loan scheme is now closed, and Queensland’s Battery Booster program ended in May 2024.
Estimating your cost: using a battery calculator
To determine the cost of a tailored solar‑plus‑battery system, many installers provide online calculators. While specific calculator steps depend on the tool, you generally need to:
- Enter your daily electricity consumption. Use your energy bills or a smart meter to determine average kilowatt‑hours per day. The Solar Choice case study used 20 kWh per day.
- Provide your location and tariff. Electricity prices and feed‑in tariffs vary by state, affecting savings and payback. In the case study, a flat rate of 40 c/kWh and a feed‑in tariff of 3 c/kWh were assumed.
- Specify solar system size or planned size. The calculator estimates how much solar generation you’ll have. A 6.6 kW array is common for small–medium homes.
- Select battery capacity and type. Try different capacities (e.g., 5 kWh, 10 kWh, 15 kWh) to see how costs and savings change. Many calculators incorporate the federal rebate and local incentives automatically.
- Review outputs: The tool will estimate installation cost, annual savings, payback period and return on investment based on your inputs and current rebate values.
Payback periods and ROI
Financial returns vary, but with the 2025 rebates the payback period for a typical battery is often between 6 and 9 years. Solar Choice’s scenario found the battery portion of the investment paid back in 7.5 years, while the overall solar‑plus‑battery system paid back in 6 years. The payback can be shorter if you:
- Use a large proportion of your stored energy (high evening consumption),
- Live in a state with high electricity prices and low feed‑in tariffs,
- Access additional state rebates or VPP revenue,
- Install the battery alongside a new solar PV system to reduce installation costs.
Summary and next steps
Solar batteries have become significantly more affordable in 2025 thanks to a generous federal rebate and falling technology costs. A 10 kWh battery plus hybrid inverter typically costs $6 000–$8 500 after the rebate and can deliver payback periods of 6 –9 years. To find the exact cost for your household, use an online battery calculator or speak with accredited installers. Compare quotes, consider your energy needs and future plans, and take advantage of available rebates to make a well‑informed decision.
FAQs
The battery‑only price covers the battery modules and installation if you already have a compatible hybrid inverter. A battery‑plus‑inverter price includes the cost of adding a new inverter to an existing system.
Many installation costs (labour, wiring, switchboard modifications) are fixed regardless of size, so spreading them over more kWh reduces the per‑kWh cost.
Yes. Installing solar and battery together allows DC‑coupling, reducing hardware and labour costs. DC‑coupled systems cost less per kWh than retrofitted AC‑coupled systems.
The federal rebate reduces the cost by roughly $345 per usable kWh. For a 10 kWh battery, that’s a discount of about $3 450. Additional state incentives can further reduce costs.
Not necessarily. Oversized batteries increase upfront cost and may never be fully utilised if your night‑time energy use is low. Right‑sizing maximises returns.
